Remove group access check if BP version = 2.1+

With the changes in https://buddypress.trac.wordpress.org/changeset/8605 (coming in 2.1), the groups loader
no longer needs to handle group access (which is great). This change set removes the check based on the version of BuddyPress that's being used.

Thanks for the great plugin!

@r-a-y, if you get a chance, could you look over this change, too?